The present invention relates to digital mobile radio communication systems and more particularly to a method for adjusting a signal quality target to be reached in power control procedures at a transceiver of a code division multiple access (CDMA) radio communication network.
Power control procedures are usually performed in CDMA radio communication systems in order to ensure a required received signal quality at a receiver with the minimum of power transmitted by a transmitter. Power control procedures comprise on the one hand the inner-loop power control and on the other hand the outer-loop power control. Inner-loop power control refers to the mechanism to make the transmitter adapt its transmitted power in order to reach a current required signal quality target. The object of outer-loop power control is to adjust dynamically the required signal quality target according to current signal propagation conditions and link quality.
In the description below, the signal quality is evaluated by means of signal quality indicators and a signal quality target to be reached is defined. Signal quality indicators as well as signal quality target are for example represented in terms of bit energy related to a spectral noise density (Eb/N0). Alternatively, another representation of these quantities can be a signal to interference ratio (SIR).
The signal preferably carries a frame structure, each frame being divided in slots. Concurrently to signal quality, frame quality is also taken into account. The frame quality is evaluated for a frame thanks to frame quality indicators and a frame quality target to be reached is also defined. Frame quality indicator as well as frame quality target are for example represented in terms of bit error rate (BER) or frame error rate (FER). Bit error rate quantity is particularly adapted for data transmission while frame error rate is more relevant for voice transmission.
A common method of adjusting a signal quality target to be reached in power control procedures consists in increasing or decreasing the signal quality target in a discrete manner by fixed steps so that the signal quality target tracks as close as possible the changing propagation conditions.
A fixed step strategy can be used if the adjustment of the signal quality target relies on a cyclic redundancy check (CRC) performed for each frame to detect frame errors. Thus, if the CRC of the currently received frame is wrong, the signal quality target is increased by a first predefined fixed step and if the CRC of the currently received frame is not erroneous, the signal quality target is decreased by a second predefined step. The first and the second predefined steps may be identical or different. However and because of their random nature, single frame errors are not representative of short term propagation conditions. An error-based decision scheme proves only reliable in the long run.
A more accurate method of adjusting the signal quality target described in prior art adopts a variable step size strategy. In that case, the adjustment is the result of a long term frame quality evaluation. A frame error rate is evaluated thanks to CRC calculation over a predefined sufficient number of frames. The step size to adjust the signal quality target is chosen according to the frame error rate evaluation. In that case an accurate frame quality evaluation requires a large number of frames, which causes a significant delay in the adjustment of the signal quality target. A substantial disadvantage is that such a method responds poorly to abrupt changes of propagation conditions.
Another disadvantage is that this method is only fitted for voice services, where end-quality is adequately described by a frame error rate. For data services, however, a bit error rate is more representative to describe the end-quality than a frame error rate.